1. Field of the Invention
The invention comprises a method for inferring transmission fluid temperature in an automatic transmission with a hydrokinetic torque converter, particularly the temperature of the transmission fluid at the flow inlet side of a transmission oil cooler.
2. Background Art
A power transmission mechanism commonly used in powertrains for automotive vehicles typically includes an internal combustion engine and a multiple ratio geared transmission with an input shaft connected to an engine crankshaft and an output shaft drivably connected to vehicle traction wheels through a differential and axle mechanism. Typically, the automatic transmission comprises a hydrokinetic torque converter, usually a three-element torque converter, located between the crankshaft of the engine and torque input gearing for the transmission.
A transmission fluid sump, sometimes referred to as an oil sump, is defined by a transmission case that envelops torque-transmitting gearing. A transmission fluid pump is drivably connected to a torque input shaft for the transmission. The pump has a fluid input port that is in fluid communication with the fluid sump. For purposes of lubrication and cooling, the pump establishes flow throughout the transmission and to the torque converter elements, including an engine-driven impeller, a turbine connected drivably to transmission gearing, and a stator located between the hydrokinetic torque converter fluid flow inlet for the impeller and the hydrokinetic fluid flow exit for the turbine. The fluid circulating through the elements of the torque converter is common to the fluid in the fluid sump for the transmission.
A transmission fluid flow outlet port (case-out port) in the torque converter communicates with a fluid cooler, typically an air-to-liquid cooler. Fluid then is returned from the cooler through a cooler return flow circuit to the torque converter and the transmission. The fluid ultimately is received by the transmission oil sump and then recirculated by the transmission pump. It is known design practice to provide a transmission fluid temperature sensor in the transmission sump for measuring fluid temperature in the sump.
As the transmission fluid circulates from the sump through the torque converter, a substantial temperature rise will occur in the toroidal flow circuit of the torque converter before the fluid is distributed from the torque converter fluid outlet to the cooler. There can be a substantial temperature difference, therefore, between the measured temperature at the transmission fluid sump and the actual fluid temperature at the fluid flow outlet port of the torque converter. The temperature rise in the transmission fluid within the torque converter can be substantial when the torque converter operates at low speed ratios as mechanical energy from the engine-driven impeller is transformed into thermal energy.
At high speed ratios for the torque converter, the transformation of kinetic energy developed by the torque converter to thermal energy is reduced. At a precalibrated engine speed for any given engine torque, a torque converter lock-up clutch will be engaged, thereby substantially eliminating a thermal energy build-up in the torque converter as fluid circulates from the fluid sump to the cooler.
It is known design practice to use a metallic transmission fluid line connecting the torque converter flow outlet port to the cooler. In installations that do not accommodate a metallic coolant flow line from the torque converter to the cooler, attempts have been made to use plastic cooler lines. A plastic material known in the automotive industry that can be used for this purpose is known as PA-12 plastic, which is a semi-flexible thermoplastic polymer. Under certain operating conditions, the fluid temperature can be high enough (e.g., about 300° F.) to cause the plastic cooler line upstream of the cooler to fail.
Typically, fluid temperatures may increase during operation of the powertrain in a stalled mode with the brakes applied at zero vehicle speeds, or when the vehicle is towing a trailer at low vehicle speed with the torque converter clutch in an open state, or during sustained high vehicle speed operation with the converter clutch in an open state. A converter clutch open state may be encountered when the converter lock-up clutch controls malfunction, whereby the converter clutch will not respond to a command by a vehicle powertrain controller to engage at a calibrated vehicle speed for a given engine throttle setting. Steps can be taken to mitigate the undesirable effects of a high temperature build-up of temperature in the oil cooler lines by truncating engine torque until the transmission fluid temperature falls below a desired temperature threshold.